Former Toro Rosso racer Buemi has filled the role for the past two seasons while Felix da Costa substituted for the Swiss at some race weekends this year.
The Portuguese had been a favourite for the Toro Rosso race seat vacated by Australian Daniel Ricciardo, who has graduated to Red Bull as quadruple world champion Sebastian Vettel's team mate, but missed out to Russian Daniil Kvyat.
"Next year sees the biggest change to the Formula One regulations for some time and the return of multiple in-season tests. To therefore be able to call on two such capable drivers is of huge benefit to the team," Red Bull principal Christian Horner said in a statement.
(Reporting by Alan Baldwin; Editing by John O'Brien)
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